This month, U.S. Special Forces Command is quietly taking delivery of a radical new drone: the Boeing A160T Hummingbird, which rewrites the rules for helicopters. Thanks to a remarkable piece of design, the Hummingbird can go further, longer, higher – and quieter – than anything else around.
According to Jane’s, 10 Hummingbirds are supposed to be delivered this month, under a joint SOCOM-DARPA program known as the Special Operations Long Endurance Demonstration (SLED).
“The Hummingbird is designed to fly 2,500 nautical miles with endurance in excess of 24 hours and a payload of more than 300 pounds. The autonomously-flown A160 is 35 feet long with a 36-foot rotor diameter,” according to Hummingbird-maker Boeing’s rather brief entry on the craft. “It will fly at an estimated top speed of 140 knots at ceilings up to 30,000 feet, which is about 10,000 feet higher than conventional helicopters can fly today.
This impressive performance is achieved by reworking traditional helicopter design. Normal helicopters work at a fixed rate of rotor revolutions per minute. Rotors are flexible, articulated and have a complex pattern of vibration; changing speed would cause potentially dangerous vibration. In addition, the rotor speed is generally as high as possible; this is an advantage when the helicopter is moving fast, so that the retreating blade still provides some lift. As a result, normal helicopters are noisy (sound is directly related to rotor speed) and wasteful, as most of the time the high rotor speed is not essential.